1. Field of the Invention
The present invention relates to image display devices such as electronic paper, flexible display devices, electronic books, and portable display devices in which electrophoretic particles are moved by the action of a driving electric field to change the display statuses of plural display pixels constituting a display image. The present invention also relates to a control method of the image display devices.
2. Description of the Related Art
Patent Document 1 discloses an image display device that encapsulates charged particles (electrophoretic particles) between a transparent display substrate and a rear-surface substrate and is capable of switching display images by separately moving the charged particles for each display pixel. In this image display device, a display driving voltage applied to each of the display pixels between the substrates is separately controlled. Accordingly, a driving electric field acting on the charged particles is changed to move the charged particles. Here, as the charged particles repeatedly perform image display, in particular, when the driving electric field continuously acts in one direction for a long period of time, the charged particles encapsulated between the substrates gradually aggregate with each other or the adhesion of the charged particles to the inner wall of a wall surface member encapsulating the charged particles gradually becomes strong. Accordingly, when the charged particles aggregate with each other or the adhesion of the charged particles becomes strong like this, the response to the driving electric field is degraded.
FIG. 1 is a graph showing results obtained by changing time (interval) until a display driving voltage is applied, so as to observe the reflectivity of a display image due to charged particles. The reflectivity under a 15-minute interval becomes lower than that under a 5-minute interval. This is mainly because the response of the charged particles to the driving electric field generated by the display driving voltage is degraded as the charged particles aggregate with each other or the adhesion of the charged particles to the inner wall becomes strong. It is found that the longer the interval is, the poorer the response of the charged particles to the driving electric field becomes.
In order to deal with this problem, the image display device of Patent Document 1 applies, before applying the display driving voltage to each of the display pixels, a preparatory driving voltage so as to generate an electric field that enables the movement of the charged particles. Accordingly, after making the charged particles easily move with the preparatory driving voltage, the image display device switches display images with the display driving voltage. As a result, even if the charged particles somewhat aggregate with each other or the adhesion of the charged particles to the inner wall becomes somewhat strong, the aggregation of the charged particles is eliminated by the preparatory driving voltage. Accordingly, the response of the charged particles to the driving electric field generated by the display driving voltage subsequently applied is improved.
Generally, when the response of the charged particles to the driving electric field is thus improved, the number of charged particles, which do not behave in accordance with the driving electric field, can be reduced. Accordingly, it is possible to properly and stably perform the display switch of an image. In addition, when the response of the charged particles to the driving electric field is improved, time required for completing the movement of the charged particles can be reduced. Accordingly, time until the display switch of an image is completed after the application of a display driving voltage can be reduced, which in turn makes it possible to perform the display switch at high speed.
Patent Document 1: JP-A-2007-33710
The image display device of Patent Document 1 can properly and stably perform the display switch of an image by improving the response of the charged particles to the driving electric field. However, it cannot perform the display switch at high speed.
Specifically, the image display device first receives a display driving instruction from the user through a switching operation for the display switch of an image and then applies the preparatory driving voltage and the display driving voltage. Accordingly, the image display device is required to ensure the time for applying the preparatory driving voltage until the time it applies the display driving voltage after receiving the display driving instruction. Therefore, even if the response of the charged particles to the driving electric field is improved by the preparatory driving voltage, the time required for applying the preparatory driving voltage is longer than the time reduced according to the improvement in the response. Thus, the display switch time until the display switch of the image is completed after the image display device applies the display driving voltage after receiving the display driving instruction becomes long. As a result, the image display device cannot perform the display switch at high speed.